Juvenile idiopathic arthritis (“JIA”), also referred to as juvenile rheumatoid arthritis (“JRA”) or juvenile chronic arthritis (“JCA”) is the most common rheumatic disease in childhood and an important cause of both short and long-term disability. The term “JIA” includes a heterogeneous group of diseases, each of which may have a variety of causes and elicit a variety of host responses. All are characterized, however, by the development of idiopathic peripheral arthritis thought to be secondary to an immuno-inflammatory pathogenesis, possibly triggered by contact with external antigen(s). JIA is classified according to three major types of disease presentation: (1) oligoarthritis; (2) polyarthritis; and (3) systemic onset juvenile idiopathic arthritis (hereinafter “SOJIA”). Each of these groups is defined by a constellation of clinical signs and symptoms during the first six months of illness.
Of the known JIA diseases, all three major groups based on primary modes of onset have been considered to: (1) have an “autoimmune etiology”; and (2) follow a complex, nonmendelian, genetic trait. Evidence of underlying autoimmunity is mainly drawn from the presence of autoantibodies and/or immune complexes in the patient's sera. Anti-nuclear antibodies occur in a high proportion of children with oligoarticular and, to a lesser extent, polyarticular onset, and these antibodies confer a higher risk of developing uveitis. Other autoantibodies directed against cytoplasmic and extracellular components such as collagen have also been described. Rheumatoid factors (RF) are detected in approximately 3% of children with JIA, and the presence of these autoantibodies defines a subgroup of polyarticular onset patients whose disease is undistinguishable from adult onset rheumatoid arthritis (Cassidy, J T and Ross, E. 2001. Textbook of Pediatric Rheumatology, 4th ed, Books on Demand Publishers, Visby, Sweden; p. 218-321).
A variety of T cell alterations have been described mainly in the blood and synovial fluid of children with oligoarthritis, but a consistent pattern of cytokine production has not been found. In both oligoarthritis and polyarthritis patients, increased soluble IL-2R and IL-1β have been found in the blood. In all three forms of JIA, TNF-α and soluble TNFαR (p55/75) have been found elevated in the serum and/or synovial fluid (SF). In SOJIA patients, abnormal expression of IL-6 has been described, especially in the blood prior to the febrile spikes, and IL-6 levels correlate with the systemic activity of the disease, with the development of arthritis, and with the increase in acute phase reactants (Yokata, S. 2003. “Interleukin 6 as a therapeutic target in systemic-onset juvenile idiopathic arthritis,” Curr Opin Rheumatol 15:581-586). IL-6 deregulation has also been proposed to mediate the linear growth retardation, thrombocytosis and anemia seen in SOJIA patients (de Benedetti F, et al., 1991. Arthritis Rheum 34:1158-1163; Mangge H, et al. 1999. J Interferon Cytokine Res 19:1005-1010; Ozen S, et al. 1997. Clin Rheumatol 16:173-178; de Benedetti F, et al., 1999. J Rheumatol 26:425-431; Muller K, et al. 1998. Br J Rheumatol 37:562-569; Shahin A, et al., 2002. Rheumatol Int 22:84-88; de Benedetti F, et al., 1992. Clin Exp Rheumatol 10:493-498; and Muzaffer M, et al., 2002. J Rheumatol 29:1071-1078).
Data on HLA segregation suggests a hereditary basis to the complex etiopathogenesis of the disease(s) in cases of oligoarticular and polyarticular onset JIA. Studies in SOJIA have yielded, however, inconsistent results. Finally, both viruses and bacteria have been described in association with chronic arthritis in children. Rubella and Parvovirus B19 can cause chronic arthritis secondary to persistence of the organism or by triggering an immune reaction, as it has also been described in cases of post-vaccination arthritis.
SOJIA represents about 10% of all the cases of JIA. The course and prognosis of SOJIA is heterogeneous, as fifty percent of patients have a monophasic course with resolution of the symptoms, while the remaining fifty percent develop a chronic relapsing and remitting course and a very severe form of polyarticular chronic arthritis. Patients with SOJIA also display an increased risk of developing hemophagocytic syndrome, a potentially fatal complication (Cassidy, J T and Ross, E. 2001. Textbook of Pediatric Rheumatology, 4th ed, Books on Demand Publishers, Visby, Sweden; p. 218-321).
Children with SOJIA present with severe systemic symptoms (fever and rash) that usually precede the development of arthritis for weeks to years. The high spiking fever, which is the hallmark of this disease, usually follows a quotidian pattern with 1-2 spikes/day. Patients look characteristically well when the fever is not present, but become quite ill with the spikes. In many patients, the fever is accompanied by a salmon-pink rash that becomes more apparent with the fever. Additionally, children with SOJIA may have hepatosplenomegaly, lymphadenopathy, pericarditis and other manifestations of serositis. These systemic manifestations may last from weeks to months and eventually tend to subside to be followed by the development of chronic arthritis. About 50% of patients will present oligoarticular involvement and will eventually recover. The other half will evolve into a polyarticular pattern, the prognosis of which correlates with the number of joints involved six months into the disease course. Up to 48% of children with SOJIA will have active arthritis ten years after the diagnosis is made (Cassidy, J T and Ross, E. 2001. Textbook of Pediatric Rheumatology, 4th ed, Books on Demand Publishers, Visby, Sweden; p. 218-321; and Lomater C, et al., 2000. J Rheumatol 27:491-496).
There are no available specific tests to establish the diagnosis of SOJIA, nor are there known prognostic indicators to ascertain its clinical course. Fever, anemia, leukocytosis and elevated erythrocyte sedimentation rate (ESR) are the main initial features of the disease, sometimes lasting several months before the diagnosis can be established. As these symptoms are nonspecific and can mimic infections, malignancies, and other diseases, patients undergo a series of very costly diagnostic tests and prolonged hospitalizations.
One of the most serious complications in patients with SOJIA is the development of hemophagocytic syndrome, also known as macrophage activation syndrome (MAS) (Cassidy, J T and Ross, E. 2001. Textbook of Pediatric Rheumatology, 4th ed, Books on Demand Publishers, Visby, Sweden; p. 218-321). The hemophagocytic syndrome, which can occur as well in the context of infectious and neoplastic diseases, is associated with serious morbidity and/or death. Its etiology, especially in the context of SOJIA, is unknown. Familial cases of MAS occur as the result of defective viral killing due to mutations in genes like perforin (involved in the release of granzyme by cytotoxic T cells/natural killer cells to target cells) or Rab27 (involved in the control of granzyme vesicle degranulation).